1. Field of the Invention
The present invention relates to an inkjet printer, a dither matrix generation method for generating a dither matrix which is used in generating a halftone image from a grayscale original image, and a recording medium in which data of the dither matrix is recorded.
2. Description of the Background Art
Conventionally used has been an inkjet printer provided with a head having a plurality of outlets, where the head moves relatively to a printing paper and printing is performed by controlling ON/OFF of ejection of fine droplet of ink from each outlet. As such a printer, known has been an apparatus where a head, in which a plurality of outlets are arranged in a direction perpendicular to a scan direction in a whole range corresponding to a width of printing paper, is provided and printing is performed at high speed with one scan (one pass) of the head relative to the printing paper.
Japanese Patent Application Laid-Open No. 2004-106248 (Document 1) discloses a technique for surely preventing occurrence of unevenness (mura) in a printed image caused by variation in ejection amounts of droplets from a plurality of outlets in an inkjet printer for printing with one pass, where a density measurement part measuring a density of each position in a width direction on a printing paper is provided and an ejection amount of droplet is corrected (i.e., the so-called shading compensation is performed) on the basis of a measured value of density in a region corresponding to each outlet in a pattern with a uniform density level (as a setting) which is printed by the head (hereinafter, the measured value is referred to as “a print density in outlet”). Document 1 further discloses a method for preventing occurrence of the above unevenness in a printed image by modifying each pixel value in a grayscale image to be written (hereinafter, referred to as “original image”) on the basis of a print density in outlet corresponding to the pixel value.
Meanwhile, in an inkjet printer, a grayscale original image has to be binarized and in the case where the dither technique is used for binarization of the original image, each pixel value in the original image and the corresponding element value in a dither matrix are compared in an electric circuit or a software integrated circuit (halftoning circuit) to perform halftoning of the original image. In such a printer, as disclosed in Document 1, when each pixel value in the original image is modified on the basis of the print density in outlet corresponding to the pixel value, a calculation operation has to be performed for multiplying each pixel value in the original image by a correction coefficient derived from the print density in outlet before halftoning of the original image and it is difficult to print an image at high speed. It is thought that the above calculation operation is performed in an electric circuit, but a special circuit (shading compensation circuit) is needed and this increases the manufacturing cost for the printer.